The present invention relates to communications techniques employing modulated waveforms. In particular, the present invention relates to a mechanism by which a receiver may automatically detect and adapt itself to the modulation that a transmitter has applied to generate a modulated information waveform.
Modern communications systems transmit staggering amounts of information. In virtually every instance, the waveform carrying the information is subjected to some form of modulation before transmission, the modulation contains the information and the carrier facilitates the transmission.
Several examples of modulation include Amplitude Modulation (AM), Quadrature Amplitude Modulation (QAM), Frequency Shift Keying (FSK), and Phase Shift Keying (PSK). The various types of modulation, and the various permutations of each type of modulation, are able to support widely differing data rates. For example, Binary PSK (BPSK) generates symbols carrying one bit of information, while Quadrature PSK (QPSK) generates symbols carrying two bits of information and 16-PSK generates symbols carrying four bits of information.
In the interest of efficiently utilizing the available spectrum, it is preferable to send as many bits of information as possible in each symbol. Thus, more expensive, higher data rate transmitters transmit waveforms using the highest order modulation they can handle. In a multi-user system, users may have different data rate requirements and may use different modulations to satisfy their needs. Furthermore, in some circumstances, it may be desirable for higher data rate transmitters to switch to a lower data rate and associated lower order modulation.
As one example, a lower data rate and lower order modulation allows the user to overcome propagation losses due to rain and blockage. Another motivation for a transmitter to adapt its modulation to use a lower data rate and lower order modulation is feedback from the receiver indicating poor channel conditions causing increased bit error rate (BER).
Transmitters generally fall into two classes: those that can only transmit one modulation, and those that may vary their modulation. In order to accommodate transmitters that can vary their modulation, it is generally preferable to enable a receiver to demodulate any of the many types and permutations of modulation that a transmitter may apply to a transmitted waveform. In the past, three general approaches have been used to allow a receiver to demodulate a number of potential modulation schemes.
In the first approach, individual frequency bands are selected from a larger spectrum of available frequencies. The individual frequency bands are then assigned as dedicated channels for each type of modulation that a received waveform may carry. Thus one frequency band may be dedicated to BPSK modulated waveforms, while a separate frequency band may be dedicated to QPSK modulated waveforms. There are many disadvantages to dedicated assignments. For example, a transmitter that needs to switch modulation schemes must make sure that bandwidth is available in the frequency band appropriate for the new modulation. A lack of bandwidth prevents the terminal from adjusting its modulation. Furthermore, dedicated assignments may waste bandwidth by providing capacity that goes unused. Thus, a BPSK frequency band may be underused because transmitters, when possible, transmit in, for example, the QPSK band to achieve higher data rates.
In the second approach, multiple demodulators designed for the various possible modulation formats operate simultaneously on the same frequency bands. These demodulators each have a measure of performance such as detected signal strength or carrier lock. The demodulator with the best performance indicator is selected as being matched to the actual signal. However, this is inefficient due to requiring multiple demodulators with only one producing valid results.
In the third approach, extra hardware, bandwidth, and power are required to enable what is generally referred to as resource control processing. In resource control processing, a receiver is configured with a demodulator that is able to switch between a predefined set of modulation schemes. Alternatively, the receiver may be configured with a set of individual unique demodulators dedicated to specific types of modulation. A required part of resource control processing is overhead control data (i.e., information that cannot be used to transmit user information), to command the demodulator in a receiver to switch demodulation methods or to enable or disable a particular modulation at a transmitting terminal. The overhead control data may be sent by the transmitter, or by another general purpose central control terminal which coordinates the transmitter (and the modulation it uses) and the receiver.
Resource control processing suffers from its own drawbacks, however. For example, useful bandwidth is wasted by sending overhead control data to a receiver (and to the transmitter to command a modulation change). Additionally, the change in modulation at the transmitter and the change in demodulation at the receiver must be synchronized to allow the receiver to accurately demodulate the modulated information waveform. A transmitter may therefore not be able to change the modulation precisely when, or for how long, it would be most efficient or necessary to change the modulation. Lack of, or loss of, synchronization can introduce errors in the demodulated waveform. Errors in the demodulated waveform may create a greater drain on bandwidth, for example if the transmitting terminal has to re-send its information. Furthermore, as noted above, extra hardware is required to process the overhead control data (and to generate the overhead control data in the first place) and adjust the demodulation scheme in the receiver. Extra hardware, of course, requires extra power. Extra power, in turn, is often in short supply, especially on board satellites.
A need has long existed in the industry for a multi-mode autonomous selection demodulator for use in communications equipment.